Liquid handling systems generally require a means for determining the level of a liquid in a tank or other liquid storage vessel. One such liquid handling system is a condensate pump for use with a heating, ventilation, and air-conditioning (HVAC) system. A conventional condensate pump has a tank or reservoir for collecting condensate from the evaporator of the HVAC system. A centrifugal pump transfers the condensate from the tank to a remote location for disposal. In a simplified embodiment of the condensate pump, the operation of the centrifugal pump is controlled by a control circuit, which turns on the centrifugal pump when the liquid level in the tank has reached a certain level and turns off the centrifugal pump when the centrifugal pump has emptied the tank. Control functions for the condensate pump may also include sounding an alarm or shutting off the HVAC system when an emergency overflow level of condensate is reached in the tank of the condensate pump. Therefore, a liquid level detection device is necessary to detect the level of liquid in the tank in order to control the operation of the centrifugal pump, an alarm, or the continued operation of the HVAC system.
In order to determine the liquid level in the tank of a conventional condensate pump, a float is often used to monitor and detect the water level in the tank. In response to movement of the float in the tank, associated float switches and a float control circuit control the operation of the electric motor driving the impeller of the centrifugal pump, trigger alarms, or shut down the HVAC system if necessary. The condensate pump float is in contact with the water in the tank and is subject to fouling from debris and algae buildup. A molded float has seams, which may fail causing the float to sink or malfunction. The float switch that is used to control the on/off operation of the electric motor is often a specialized and costly bi-stable snap-action switch. A conventional condensate pump, which incorporates a safety HVAC shut off switch and/or an alarm switch in addition to the motor control switch, may have a separate float or linkage to operate the HVAC shutoff switch or the alarm switch further complicating the condensate pump. Further, a conventional condensate pump often requires a float mechanism retainer to prevent shipping damage, and the float mechanism retainer must be removed prior to pump use.
The prior art also includes capacitive sensors to sense the level of the water in the tank of the condensate pump to control the operation of the pump motor, to trigger alarms, or to shut down the HVAC system if necessary. In one conventional capacitive water level sensor, at least one of the capacitance plates of the capacitive sensor is in contact with the water in the tank in order to produce a detectable change in capacitance as the water contacts or exposes the capacitance plate. Capacitance plates that are in contact with the water in the tank are subject to fouling based on the buildup of debris or algae. The fouling of the capacitance plate adversely affects the performance of the capacitive sensor.
Prior art capacitance sensors are bi-directional in nature. Sensing plates or electrodes are connected to an oscillator circuit, an RC or LC timing circuit (where the C capacitive component varies in relation to the proximity between the sensed fluid and the sensor plate), or other circuit arrangement which facilitates the measurement of timing or frequency by forcing a change of charge onto or off of the sensor plate or electrode and measuring the time required to reach a certain threshold potential or oscillation frequency. In order to build capacitance sensors of a reasonably small scale the resultant capacitance values typically range between a few and a few hundred picofarads and therefore require circuitry with high sensitivity and high input impedance. These high impedance circuits work well to detect water and other fluids, but due to their sensitive nature are subject to minute leakage currents created by dirty insulators, slime and mineral buildup in the vicinity of the sensing electrodes. A high impedance sensor may also fail from buildup which functions to connect it to an adjacent sensor. Additional failure modes can occur from stray radio frequency fields, electrostatic buildup and other outside electromotive forces which can easily influence the high impedance capacitive sensor input.
In another prior art capacitive sensor, the capacitance plates are mounted outside of the tank and not in contact with the water in the tank. In order to sense accurately the water level, such prior art external capacitive sensors have a first capacitance plate extending the height of the tank and one or more additional capacitance plates position at anticipated transition points along the height of the tank in order to determine when the water level has reached one of the transition points. Such additional capacitance plates are deemed necessary in order to offset the effects of deposits that may form on the inside of the tank adjacent to the external capacitance plates thereby affecting the capacitance value. Further, the capacitance plates of the capacitive sensors typically represent a high impedance inputs to the control circuit. Because of the high impedance, the capacitance plates pick up extraneous background interference that can further affect the accuracy of the capacitive sensors.
Yet another prior art sensor includes a sonic detector for determining the level water in the tank. Such a sonic detector includes a sonic generator that emits a sound wave from above the water toward the water. The sonic detector further includes a receiver for receiving the echoed sonic wave as the sound wave bounces off of the water in the tank. By measuring the time between the transmission of the sonic wave and the reception of the echoed sonic wave, the water level in the tank may be determined. Such sonic detectors are difficult to calibrate and may be affected by extraneous sound waves such as those created by the pump motor.